Rust Error handling Common Result methods
Example
use std::io::{Read, Result as IoResult};
use std::fs::File;
struct Config(u8);
fn read_config() -> IoResult<String> {
let mut s = String::new();
let mut file = File::open(&get_local_config_path())
// or_else closure is invoked if Result is Err.
.or_else(|_| File::open(&get_global_config_path()))?;
// Note: In `or_else`, the closure should return a Result with a matching
// Ok type, whereas in `and_then`, the returned Result should have a
// matching Err type.
let _ = file.read_to_string(&mut s)?;
Ok(s)
}
struct ParseError;
fn parse_config(conf_str: String) -> Result<Config, ParseError> {
// Parse the config string...
if conf_str.starts_with("bananas") {
Err(ParseError)
} else {
Ok(Config(42))
}
}
fn run() -> Result<(), String> {
// Note: The error type of this function is String. We use map_err below to
// make the error values into String type
let conf_str = read_config()
.map_err(|e| format!("Failed to read config file: {}", e))?;
// Note: Instead of using `?` above, we can use `and_then` to wrap the let
// expression below.
let conf_val = parse_config(conf_str)
.map(|Config(v)| v / 2) // map can be used to map just the Ok value
.map_err(|_| "Failed to parse the config string!".to_string())?;
// Run...
Ok(())
}
fn main() {
match run() {
Ok(_) => println!("Bye!"),
Err(e) => println!("Error: {}", e),
}
}
fn get_local_config_path() -> String {
let user_config_prefix = "/home/user/.config";
// code to get the user config directory
format!("{}/my_app.rc", user_config_prefix)
}
fn get_global_config_path() -> String {
let global_config_prefix = "/etc";
// code to get the global config directory
format!("{}/my_app.rc", global_config_prefix)
}
If the config files don't exist, this outputs:
Error: Failed to read config file: No such file or directory (os error 2)
If parsing failed, this outputs:
Error: Failed to parse the config string!
Note: As the project grows, it will get cumbersome to handle errors with these basic methods (docs) without losing information about the origin and propagation path of errors. Also, it is definitely a bad practice to convert errors into strings prematurely in order to handle multiple error types as shown above. A much better way is to use the crate error-chain.
